1. Field of the Invention
The present invention relates generally to methods and apparatus for fabricating electronic devices on substrates, including integrated circuits. More particularly, the present invention relates to methods for selectively or preferentially depositing material in trenches, holes or vias formed in a substrate surface using physical vapor deposition (PVD) with a high density plasma
2. Background of the Related Art
Sputtering or physical vapor deposition is a known technique for depositing thin films onto a substrate. Typically in a sputtering system, a glow discharge plasma is generated in an atmosphere of inert gas by maintaining an inert gas atmosphere at a low pressure between a two electrode system comprising a cathode and an anode. A plasma is created in the space between the electrodes in which the inert gas atmosphere becomes at least partially ionized. The plasma is bounded from each of the electrodes by a plasma sheath or dark space. A high negative potential applied to the cathode (with respect to the plasma) then accelerates ions from the plasma onto the cathode (usually the target or desired coating material) where cascade lattice collisions cause the ejection of neutral target atoms. At gas pressures below about 10 milliTorr, the ejected or sputtered atoms travel across the plasma, essentially without interaction, and deposit on the anode which may comprise the substrate or wafer to be coated. At higher gas pressures from about 10 milliTorr to about 50 milliTorr, the sputtered neutral atoms can interact with the plasma while traversing therethrough. This results in a nearly isotropic directional distribution of the sputtered material at the surface of the wafer or substrate.
The surface of the wafer being coated is typically patterned and frequently includes high aspect ratio apertures and vias wherein the ratio of depth to width is greater than 4:1. Apertures or vias are formed in a substrate to provide a conductive path (trench) or contact (hole) between film layers. PVD is a preferred technique for coating substrates with metals. However, as geometries shrink it becomes more difficult to fill high aspect ratio features without forming voids in the feature. As deposition material is deposited in the aperture and on the field of the substrate, the aperture is closed in a phenomenon known as "crowning" where the deposited material closes the top of the aperture before it has been completely filled. Material is deposited on the areas between the holes and trenches, commonly referred to as the "field" of the wafer, and closes the openings of the small features. Generally, it is desired to have little or no metallic or conductive coatings on field areas of the wafer, at least in the same plane as the top of the trenches or holes forming conductive paths or contacts. The build up of material on the field adjacent a hole or trench leads to the problem of "shadowing" in which an overhanging edge of such material can shield or deflect sputtered material from reaching the sidewalls and floor of the hole or trench. Thus, it is difficult to selectively or preferentially fill the trenches, vias, or holes without covering the field areas.
Ionized PVD uses a high density plasma between a sputtering target and a substrate to ionize a high fraction of sputtered atoms before depositing onto the substrate. These ions are accelerated perpendicularly towards the surface of the substrate within a plasma sheath, improving the selective or preferential filling of high aspect ratio holes, trenches, or vias.
Biasing of the substrate relative to plasma potential is widely used in ionized PVD to control the energy of ions reaching the substrate and improve results . The bias voltage can be applied directly with a conductive substrate or a conductive layer on the substrate. With a non-conductive or highly resistive substrate, RF bias voltage can be capacitively coupled to the substrate. The frequency of the RF bias is typically between 10 Kilohertz and 300 Megahertz depending on the time duration in which the substrate surface is significantly charged by the electric currents from plasma. However, conventional sinusoidal RF bias does not accurately control the energy of ions reaching the substrate surface because of variations in bias voltage during each cycle.